Suspended in the air

While walking along Bradford Street I couldn't take my eyes off the enormous amount of concrete suspended by this crane. I've seen this before, so it's obviously nothing unusual, but does anyone know why cranes are left overnight with concrete blocks hanging in the air. I find it fairly scarey to be truthful, but there has to be a reason for it. Could it be to stabilise the crane or balance it in some way? Can anyone enlighten me on the subject?